In valve drives for internal combustion engines with a decompression brake—usually large-volume diesel engines for use in utility vehicles—provision is made for one or all of the outlet valves of a cylinder to be re-opened during the course of the compression stroke in the engine braking mode in order to considerably increase the charge exchange work for the benefit of negative engine power. In relation to the regular outlet valve actuation in the expansion stroke, the re-opening of the outlet as a function of the control times thereof requires significantly greater valve actuating forces, since the outlet valves must open counter to the compression pressure, and occasionally counter to the final compression pressure in the cylinder.
As proposed in U.S. Pat. No. 7,392,772 B2, which is considered to be generic, it is possible in valve drives of said type for the compensation of the valve play to also take place in an automatic and continuously variable fashion by means of a hydraulic valve play compensating element, wherein it is intended for the valve play compensating element to be arranged as an alternative to an adjusting screw, which mechanically adjusts the valve play, between the valve-side lever section of a rocker arm, which is disclosed in said document, and the gas exchange valve. Said positioning of the valve play compensating element can, however, pose problems since the high valve actuating forces during the engine braking mode cause an excessive sinking of the valve play compensating element on account of hydraulic medium being FORCED out of the high-pressure chamber of said valve play compensating element. The regeneration of the valve play compensating element, which is subsequently required, is also hindered by the multiple opening of the outlet valve during a cam rotation, since only a comparatively small zero-stroke cam angle is available for the re-induction of hydraulic medium into the then-expanding high-pressure chamber. One result of this imbalance between the sinking and regeneration of the valve play compensating element can, depending on the duration of the engine braking mode, be a successive complete collapse of the valve play compensating element into its mechanical blocked position, wherein this may be associated both from a thermodynamic aspect and also from a mechanical aspect with inadmissible truncation of the cam elevations for the expansion stroke and the engine braking mode. With regard to the thermodynamics, reference is made to the inadequate charge exchange on account of excessively small valve opening cross sections, and with regard to mechanics, reference is made to the high lift-up and set-down speeds of the gas exchange valves on account of a lack of cam ramps.
Said considerations apply correspondingly to the valve drive proposed in DE 10 2006 031 706 A1, which valve drive has a hydraulic valve play compensating element arranged between a rocker arm and the gas exchange valve.